The present invention relates generally to a method for driving a liquid-crystal display panel and more particularly to a method for driving an active matrix type liquid-crystal display panel of the type in which switching transistors are connected to picture elements of the liquid-crystal display panel.
Well known in the art are the matrix type liquid-crystal display panels in which each display unit or element is provided with a switching transistor in order to display a numeral, letter or other kind of image.
Such liquid-crystal display panels as described above have an equivalent circuit as shown in FIG. 1. For instance, each display unit or element comprises one of thin film transistors 4a-4c made of amorphous silicon and one of liquid-crystal unit cells 5a-5c. The thin film transistors 4a-4c are formed over a glass substrate by a thin film formation technique. The gates of the transistors 4a-4c are connected to scanning electrodes 2a-2c, respectively, and the sources are connected to a signal electrode 1. The drains are connected to one of the terminal electrodes of the liquid-crystal unit cells 5a-5c, respectively. The other terminals of the liquid-crystal unit cells 5a-5c are connected to a common electrode 3 which is disposed in opposed relationship with the glass substrate, the liquid-crystal being sandwiched between the glass substrate and the common electrode 3. Such a liquid-crystal display panel as described above is operated by an AC driving method as will be described below with reference to FIGS. 2-5.
FIG. 2(a) shows a voltage waveform applied to the common electrode 3; FIG. 2(b), a voltage applied to the signal electrode 1; FIGS. 3(a), 4(a) and 5(a), voltage waveforms applied to the scanning electrodes 2a, 2b, and 2c, respectively; and FIG. 3(b), 4(b) and 5(b), voltages V applied to the liquid-crystal unit cells 5a, 5b and 5c, respectively.
When a voltage V.sub.G is applied to the gate of the thin-film transistor 4, the transistor 4 is turned on so that the voltage between the electrodes sandwiching the liquid-crystal approaches a voltage which is the difference between the potentials of the signal electrode 1 and the common electrode 3. In this case, in order to ensure a high quality image, this voltage must be maintained substantially at a predetermined level even after the voltage applied to the scanning electrode 2 is removed so that the thin-film transistor 4 is turned off. This must be maintained until the subsequent cycle (display cyclic period) when the content of the subsequent display is changed.
With such a display panel as described above, the so-called matrix display can be effected by selecting the transistors through the scanning electrodes 2a-2c, but it has the following defects.
As is well known in the art, a thin-film transistor has an electrostatic capacitance C.sub.GD between the drain and the gate electrodes and a liquid-crystal unit cell can be regarded as a capacitor. Therefore, if the liquid-crystal unit cell is assumed to have a capacitance C.sub.LC, an equivalent circuit of one picture element or pixel of the liquid-crystal display panel is as shown in FIG. 6 when the thin-film transistor is turned off. At the points of time when the potentials of the common electrode 3 changes (T.sub.1 -T.sub.4 in FIGS. 2-5), the transistor is turned off so that the equivalent circuit as shown in FIG. 6 represents one picture element or pixel of the liquid-crystal display panel. In FIG. 6, when the potential of the common electrode 3 changes by .DELTA.Vc, the voltage across the capacitor C.sub.LC changes by .DELTA.V which is given by the following equation (1). ##EQU1## The polarity of the voltage .DELTA.V is the same as that of the voltage .DELTA.Vc. As a consequence, as shown in FIGS. 3-5, the voltage .DELTA.V always functions to decrease the absolute value of the voltage V.sub.LC across the capacitor C.sub.LC. This means that the effective value of the voltage applied to the liquid-crystal is decreases by the voltage .DELTA.V.
When the decrements in the effective values of the voltages applied to the liquid-crystal unit cells 5a, 5b and 5c are assumed to be .DELTA.V.sub.EFa, .DELTA.V.sub.EFb and .DELTA.V.sub.EFc, the following relationship is present as is apparent from FIGS. 3-5. EQU .DELTA.V.sub.EFa &lt;.DELTA.V.sub.EFb &lt;.DELTA.V.sub.EFc ( 2)
And we may consider that EQU .DELTA.V.sub.EFa .apprxeq.0 (3)
and EQU .DELTA.V.sub.EFC .apprxeq..DELTA.V (4)
This means that even when the same signal voltage is applied to each of the picture elements of the liquid-crystal display panel, there exists a difference in effective value of applied voltage between the picture elements to which are applied the scanning voltages at different points of time. As a result, there is a difference in light transmissivity between picture elements so that a luminance gradient occurs over the whole picture.
In the liquid-crystal display panels, the scanning voltage is applied sequentially from the leftmost scanning electrode so that, as is apparent from FIGS. 3-5, the closer to the leftside a picture element is, the higher a light transmissivity it has. When the following values are assumed:
C.sub.LC =1 PF, PA1 C.sub.GD =0.05 PF and PA1 .DELTA.V.sub.C =10 V,
then, from Eq. (1) and Eq. (4), the voltage decrements are calculated as follows: EQU .DELTA.V=0.48 V
and EQU .DELTA.V.sub.EFc =0.48 V
When the gradational display is effected by changing the voltage applied to the signal electrodes by a step of 0.24 V, for example, the voltage of 0.48 V is equivalent to the voltage representing two steps of gradation. As a result, it is impossible to effect a practical gradational display. FIG. 7 shows the relationship between the location of a picture element on the panel obtained by the simulation under the above-described conditions and the effective value of the voltage applied across the liquid-crystal (the signal voltage is 6.5 V) at the picture element. The ordinate represents the voltage while the abscissa represents the location on the liquid-crystal panel where the leftmost end is represented by 0 and the rightmost end is represented by 1. It is seen that the effective voltage value changes almost linearly relative to the location.